There are many applications which require a coupling to couple a device to a shaft. Often, such couplings allow rotational movement of a shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device. In certain application, the coupling must substantially eliminate hysteresis (which is the lagging of a physical effect on a body behind its cause) in rotational movement of the shaft with respect to the device.
One application of the coupling is a torque sensor device which determines a torque input on a shaft which comprises a torsion bar longitudinally connected to a primary bar. The primary bar is relatively inflexible to a torque input, while the torsionally bar torsional flexes to a torque input. The magnitude of the torque input may be determined by measuring the rotation of the torsion bar relative to the primary bar. One of the difficulties in coupling the torsion bar to the primary bar is that the torsion and primary bar may not be coaxial due to manufacturing tolerances or design requirements. External forces acting perpendicularly to the longitudinal axis of the torsion bar may also cause translation of the torsion bar in an x-y direction of an x-y plane (which is perpendicular to the longitudinal axis of the torsion and primary bar) such that the torsion bar may become temporarily non-coaxial with the primary bar. In addition to connecting with non-coaxial bars, the coupling should transmit the relative rotation of the torsion bar to the torque sensor device without backlash. In other words, the coupling allows rotational movement of the shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device. Thus, the coupling substantially eliminates hysteresis in rotational movement of the primary bar with respect to the torque sensor device.
The torque sensor device may be used to accurately measure the input torque acting on a steering column shaft in an electronic power steering system of an automobile or truck. In this application, an input torque acts on the steering column shaft when an operator turns the steering wheel. The steering column shaft includes the primary bar and the torsion bar. The rotation of the torsion bar relative to the primary bar may be measured with a potentiometer. The torque sensor device may include a coupling which couples the torsion bar to the primary bar so that a sensor brush may slidingly contact a variable resistor as the torsion bar rotates relative to the primary bar. In order to accurately determine the relative rotation of the torsion bar, the coupling should accurately transfer the relative rotation of the torsion bar to the sensor brush with substantially no hysteresis and still allow the translation of the torsion bar in the x-y plane.
Several devices are currently available which couple non-coaxial shafts. However, none of the devices thus far appear to be without problems. One attempt to satisfy the needs discussed above is disclosed in U.S. Pat. No. 3,834,182 (Trask et al.). Referring to FIGS. 1 and 2, this patent describes a flexible coupler 20 for connecting nominally coaxial shafts drivingly connected to one another. The coupler 20 permits a limited amount of axial misalignment between the shafts. The coupler 20 comprises three basic elements: an enlarged cylindrical hub 22 fixed to a first shaft, a second smaller cylindrical flange 26 fixed to another shaft 28 in juxtaposition to the hub 22, and a "floating" annular ring 30 also juxtaposed with the hub 22 about the flange 26. Loose fitting complementary axial lugs 32 and notches 34 interconnect the hub 22 and ring 30, and loose fitting complementary radial lugs 36 and notches 38 are interfitted between the ring 30 and flange 26. The flange 26 and ring 30 are located relative to the hub 22 for axial clearance, permitting limited angular misalignment between the two shafts 24, 28. The flange 26 and circumjacent ring 30 form a planar surface juxtaposed with the inner planar surface 40 of the hub 22. However, due to the loose fitting complementary axial lugs 32 and notches 38 and the loose fitting complementary radial lugs 36 and notches 38, gaps 42 between the lugs 32 and notches 34 may lead to rotational play between the first and second shaft 24, 28.
U.S. Pat. Nos. 2,956,187 (Wood), 3,859,821 (Wallace), 4,357,137 (Brown), and 4,464,141 (Brown) appear to provide a coupling with less rotational play between a first and second shaft than the Trask patent. These patents describe a flexible coupling for transmitting power from a drive shaft to a driven shaft. The coupling includes a primary coupling member having a hub section for receiving and rotating with a first shaft, a flange section having a resilient insert therein, and a secondary coupling member located centrally within the resilient insert for receiving and rotating with a second shaft. The resilient insert is interference fitted into the primary coupling member, and the secondary coupling is interference fitted into the central region of the resilient insert. The resilient insert is adequately flexible to allow for axial misalignments between the shafts. However, a slight rotational play appears to exist between the first and second shafts because the resilient insert flexes to an input torque acting on the shafts.
Another coupling with reduced rotational play is disclosed in U.S. Pat. No. 3,728,871 (Clijsen) which describes a coupling for connecting two approximately registering shafts. Referring to FIGS. 3 and 4, the coupling 50 comprises two connecting pieces 52, 54 respectively connected to a first 56 and second shaft 58. A loose coupling disc 60 is fitted between the two connecting pieces 52, 54 and couples the rotary movements of both connecting pieces 52, 54 to each other and has a limited play in two mutually perpendicular radial directions with respect to the individual connecting pieces 52, 54. Play in the direction of rotation is reduced by a resilient C-shaped spring member 62. One drawback of this coupling 50 appears to be that it is relatively complicated. This may result in an increase in manufacturing time and cost due to the numerous precision shaped components required. It also may result in a less reliable device because the inclusion of more components may translate into a statistically less reliable device.
Another coupling with reduced rotational play is a conventional Oldham coupling. Referring to FIG. 5, the Oldham coupling 100 comprises three basic elements: a first member 102 connected to a first shaft at one end and having an axially extending tongue 104 at the other end, a second member 106 connected to a second shaft at one end and an axially extending tongue 108 at the other end, and a third member 110 positioned between the first 102 and second member. The third member 110 has a groove 112 at each end which slidingly mates with the respective tongues 104, 108. One drawback of the Oldham coupling 100 is that it appears to be relatively complicated. For the same reasons discussed above in regards to the Clijsen patent, the Oldham coupling may not satisfy certain needs for the torque sensor device.
Thus, there remains a need for a coupling that allows rotational movement of a shaft to be transferred to a device while not allowing translational movement of the shaft to be transferred to the device in an inexpensive, reliable, and rugged manner.